1. Field of the Invention
The invention relates to a retaining device with a multi-layer protective cladding for protecting the bearing and containment structure of a spreading chamber, for the controlled spreading and cooling of a hot melt, in particular a core melt, which after emerging from the nuclear reactor pressure vessel of a nuclear reactor installation designed on the spreading concept can be carried into a spreading chamber and cooled therein.
2. Description of the Related Art
It has been known heretofore, for retaining core melt emerging from a reactor pressure vessel of a nuclear reactor installation, to dispose a crucible-like catching container directly below the reactor pressure vessel, in the reactor cavern carrying the reactor pressure vessel. The catching container can hold the core melt within a compact volume and can be cooled both on its outside and in its interior. British Patent 2 236 210 A describes one such retaining device with a catching container, also called a core catcher. The core catcher is embodied by a supporting structure that is spaced apart from the wall and bottom of the reactor cavern and is supported by support elements disposed on the bottom. In the interior, the core catcher is lined with a core retention layer of zirconium oxide blocks. The core retention layer is lined with a sacrificial layer, preferably of a steel, for its protection during normal power plant operation. The core retention layer is thus sandwiched between the bearing structure and the sacrificial layer. For mechanical stability of the sacrificial layer, the zirconium oxide blocks are joined to one another and to the bearing structure and to the sacrificial layer by a zirconium oxide cement. To enable the core catcher to hold all the emerging core melt, the container is drawn upward in crucible fashion to beyond the bottom of the reactor pressure vessel. As a result the core melt can be caught in a kind of smelting crucible with a small directly-coolable surface area.
A retaining device for receiving a spreading core melt is described in the conference offprint "Emerging Nuclear Energy Systems 1989, ICENES '89, Karlsruhe July 3-6, Proceedings of the Fifth International Conference on Emerging Nuclear Systems, pp. 19-24; see particularly FIG. 1 on page 23 in conjunction with the text on page 21. In that known retaining device, a cooled catch basin (core catcher) is likewise disposed inside the reactor containment directly below the reactor pressure vessel; in this basin, the melt can spread over a large area and can be cooled with water in direct contact with an enlarged surface area. The steam generated by the decay heat of the melt condenses in the upper part of the steel shell of the reactor containment and from there flows back to the retaining device. Specifically, the bottom of the catch basin is supported by a support structure made up of double-T beams, and is formed by a perforated steel plate. Core melt can be cooled directly with water both through the steel plate and via openings in the wall of the catch basin. What is known as a "sacrificial concrete layer" is located on the steel plate. That sacrificial layer is reinforced in the surface region of the shielding pit, or in other words below the reactor pressure vessel. Cooling water in the lower part of the reactor containment permanently surrounds the catch basin. The cooling water can also spread along the top of the sacrificial layer, or in other words the surface of the sacrificial layer toward the reactor pressure vessel. However, as much as possible, an immediate contact of a core melt emerging from the rounded bottom portion of a reactor pressure vessel with water should be avoided. Moreover, in the known retaining device the water layer below the catch basin is the only protective layer for the structural concrete located beneath it.